Electrostatic chuck with improved RF power distribution

ABSTRACT

A susceptor for a wafer support of a semiconductor processing chamber having multiple parallel electrical contacts between an RF electrode and a thick robust electrode near a bottom of the susceptor. The thick robust electrode has a low resistance and, therefore, evenly distributes RF power over its area. The multiple parallel contacts ensure that the RF power is also uniformly distributed across an area of the RF electrode. A plurality of electrically conductive vias extending between the robust electrode and the RF electrode make a plurality of parallel electrical contacts therebetween. Generally, the robust electrode is attached to a bottom side of the susceptor and is aligned substantially parallel to the RF electrode. An insulator plate is attached to a bottom of the susceptor for electrically isolating the robust electrode for the pedestal.

BACKGROUND OF THE DISCLOSURE

1. Field of the Invention

The invention relates to semiconductor wafer processing equipment and,more particularly, the invention relates to electrostatic substratesupports having an RF bias electrode.

2. Description of the Background Art

A semiconductor wafer processing system for manufacture of integratedcircuits (IC's) generally includes a vacuum chamber within which ismounted a wafer support during processing. The wafer support typicallycomprises a susceptor mounted to a pedestal. The pedestal is typicallyfabricated from a metal such as aluminum. The susceptor may befabricated from laminated sheets of a polymer. However, for hightemperature applications, the susceptor is typically fabricated from aceramic material such as aluminum oxide or aluminum nitride. Thesusceptor typically contains various components which provide heatingand/or cooling of the wafer. The susceptor may also include elements forclamping (chucking) a wafer to retain it in a stationary position uponthe susceptor surface. Such clamping is provided by either a mechanicalclamp or an electrostatic chuck. The susceptor may also include one ormore electrodes for applying a bias voltage to the wafer. Such a biasvoltage may be a direct current (DC) bias or a radio frequency (RF)bias. An RF bias may be used, for example, to supply or enhance power toa plasma that exists within the chamber during an etch or depositionprocess.

FIG. 1 depicts a wafer support 100 of the prior art. In the wafersupport 100, a pedestal 102 supports a ceramic susceptor 104. Thesusceptor 104 is typically made by cold laminating several layers 106_(i) (e.g. layers 106 ₁, 106 ₂ . . . 106 ₅) of “green tape” consistingof a ceramic powder of alumina or aluminum nitride with a suitableorganic binder such as butadiene (synthetic rubber) or poly-methylmethacrylate. The electrode patterns 108 are screen or stencil printedonto the appropriate green tape layer using inks or pastes made withmolybdenum or tungsten powders along with a suitable organic binder.Interconnections between the electrodes 108 and connections 112 to theexterior of the susceptor are made by punching holes in the green tapelayers and filling them with the same or similar tungsten/molybdenumpaste through screen printing masks to form vias 110. The laminatedlayers are sintered at elevated temperatures to solidify the ceramic toform the monolithic susceptor 104 with thick film metal electrodes 108embedded in the ceramic. During the sintering process, the organicbinders are charred and removed as CO and CO₂.

The metal paste and the ceramic layers 106 generally sinter at differenttemperatures and, thus, shrink non-uniformly during sintering. Suchnon-uniformity in the shrinkage, if severe enough, causes severe bowing,distortion or cracking of the laminate. To alleviate such problems, themetal inks or pastes are usually mixed with large proportions (up to 40%by volume) of ceramic powder to match the shrinkage behavior of thesurrounding ceramic during sintering.

Prior art attempts to solve the problem include using thickerelectrodes. However, the thickness of the electrodes 108 is limited toonly 10 or 15 microns by the sintering process. If the metal electrodesare made thin and fragile they easily break and reform many times due tothe shrinkage in the surrounding ceramic. Thus, strains are dissipatedbefore they become large enough to damage the ceramic.

The resulting electrodes 108 and interconnecting vias 110 andconnections 112 are, therefore, highly resistive. High resistivity isnot a problem where the electrode 108 serves only as a DC electrode forchucking or DC bias. However, high resistivity presents a problem whenit is desired to use the electrode 108 for RF bias.

The problem arises because, in common designs, the vias 112 connectingthe electrodes 108 to the outside are located in the central area 114 ofthe susceptor. A thin highly resistive electrode will have a highimpedance due to ohmic resistance. Consequently, a large proportion ofthe power delivered to the electrodes 108 will be dissipated as heat.Thus, the efficiency of RF power transferred to the plasma in thechamber is low. This is unsuitable for delivery of RF energy to theplasma at power levels of 500 to 1500 watts. Furthermore, the RF powerdistribution over the area of electrodes 108 is non-uniform which leadsto non-uniformity of the plasma temperature across the wafer. The plasmadevelops “hot spots” where the plasma temperature is greater.Consequently, the etch, deposition or other plasma process will benon-uniform over the surface of the wafer. Thus, a number of the IC's ona given wafer may be rendered unusable thereby decreasing wafer yield.

Other solutions involve stacking multiple thin RF electrodes in layersseparated by layers of ceramic. The RF power is then capacitivelycoupled from one electrode layer to the next. Although this does reducethe overall impedance of the metal structure, it also leads to powerlosses through the capacitive couplings. Furthermore, it does not solvethe problem of non-uniform RF power distribution over the area of theelectrodes.

Therefore, a need exists in the art for a susceptor for a semiconductorprocessing system having a low impedance electrode structure thatuniformly distributes RF power over the area of the electrode and aconcomitant method of manufacturing same.

SUMMARY OF THE INVENTION

The disadvantages associated with the prior art are overcome by thepresent invention of a susceptor having multiple parallel electricalcontacts between an RF electrode and a thick, robust electrode near thebottom of the susceptor. The thick, robust electrode has a lowresistance and, therefore, evenly distributes RF power over its area.The multiple parallel electrical contacts ensure that the RF power isalso uniformly distributed across an area of the RF electrode. Thesusceptor generally comprises a ceramic support body, at least one RFelectrode embedded within the body, a robust electrode disposed belowthe RF electrode, and a plurality of electrically conductive viasextending between the robust electrode and the RF electrode. The viasmake a plurality of parallel electrical contacts between the robustelectrode and a plurality of points substantially uniformly distributedover an area of the RF electrode. The robust electrode, RF electrodesand vias are typically made of a metal such as molybdenum, tungsten orcopper. Generally, the robust electrode is attached to a bottom side ofthe support body and is aligned substantially parallel to the RFelectrode. An insulator plate is attached to the bottom side of thesupport body for electrically isolating the robust electrode from apedestal that supports the susceptor.

A method of fabricating a susceptor of the present invention begins withthe step of forming a ceramic body. The body is formed by creating aplurality of layers of ceramic green tape and stacking the layers on topof one another. One or more electrodes are embedded within the ceramicbody by screen printing one or more electrode patterns on selected onesof the layers using a paste containing a metal powder. A plurality ofvias is formed by punching holes in selected layers such that the holesin adjacent layers are aligned when the layers are subsequently stackedtogether. After the holes are filled with a paste containing a metalpowder the layers are stacked. After stacking, the layers are cured toform the ceramic body. The metal powder consolidates to form theelectrodes and vias. The vias form a plurality of contacts at aplurality of points substantially uniformly distributed over an area ofthe RF electrode.

A robust electrode, having a low resistance, is disposed below the otherelectrodes at a bottom of the ceramic body. The robust electrode isjoined to the exposed ends of the vias. Thus, the vias form a pluralityof parallel electrical connections between the robust electrode and aplurality of points distributed over the area of at least one RFelectrode in the susceptor.

The low resistance robust electrode and multiple parallel electricalconnections to the RF electrodes in the susceptor of the presentinvention provide a more uniform distribution of RF power over the areaof the RF electrode. This leads to a more uniform plasma temperatureand, hence, more uniform processing of wafers. Furthermore, the overallstructure has a low impedance and consequently there is efficienttransmission of RF energy from the robust electrode to the RF electrodeand from the RF electrode to the plasma with minimal power loss.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 depicts a typical ceramic susceptor of the prior art;

FIG. 2 depicts a wafer support that employs a susceptor of the presentinvention; and

FIG. 3 depicts a flow diagram of the method of making the susceptor ofthe present invention.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION

A wafer support 200 employing a susceptor 204 of the present inventionis depicted in FIG. 2. The wafer support is, for example, situatedwithin a process chamber 201 of a semiconductor wafer processing system.The wafer support includes a pedestal 202 that supports the susceptor204. The susceptor generally includes a support body 205, a robustelectrode 212, an RF electrode 208, and a plurality of conductive vias210. The conductive vias 210 make parallel electrical connectionsbetween the robust electrode 212 and the RF electrode 208. The supportbody 205 is, for example, comprised of a plurality of layers 206 _(i)(e.g. layers 206 ₁, 206 ₂, 206 ₃, 206 ₄) of ceramic material such asaluminum oxide (Al₂O₃ or alumina), aluminum nitride or similar material.The support body 205 includes a support surface 207 for supporting asubstrate, such as a semiconductor wafer (not shown). The supportsurface 207 may be flat or contoured as necessary for properlysupporting the substrate. Furthermore, the support surface 207 mayinclude grooves or channels for backside gas cooling.

The robust electrode 212 is disposed near a bottom 209 of the supportbody 205. The robust electrode 212 is preferably made from a lowresistivity metal such as a sheet of copper, molybdenum or tungsten. Therobust electrode 212 should be thick enough to permit easy handling butnot too thick so that it is not insulated from plasma at its sides. Aconvenient thickness range for the robust electrode 212 is betweenapproximately 0.005 inches to approximately 0.025 inches. Alternatively,the robust electrode 212 could be screen printed or stencil printed ontoone of the ceramic layers 206 _(i) such as layer 206 ₄, but only afterthe support body 205 has been fully densified by sintering. In thiscondition the shrinkage problems are obviated. However, electrodesthicker than 0.005 inches (125 microns) are difficult to make due to thelimitations of the screen printing process. In fact, stencil printing ispreferred for a thickness above 50 microns.

In the example shown in FIG. 2, the robust electrode 212 has an areathat substantially corresponds to an area of the susceptor 204. Aninsulator plate 214, disposed below the robust electrode 212,electrically insulates the robust electrode from the pedestal 202. Theinsulator plate 214 contains a central opening 217 to allow forconnection between the robust electrode 212 and DC or RF voltagesources.

The RF electrode 208 is disposed, for example, proximate the supportsurface 207 and situated between two of the ceramic layers such aslayers 206 ₁ and 206 ₂. The RF electrode 208 is used to apply RF biasvoltages to the substrate on the wafer support surface 207. The RFelectrode 208 may also be used to apply a DC bias to the substrate. TheRF electrode is typically made of tungsten or molybdenum. Although onlya single RF electrode 208 is shown for the sake of clarity, thesusceptor 204 of the present invention may include any number andarrangement of RF electrodes. For example, multiple parallel RFelectrodes may be stacked on top of one another separated by layers ofceramic. Alternatively, multiple electrodes may be placed side by sidebetween the same ceramic layers. Furthermore, some combination ofstacked and side-by-side electrodes may be employed.

In addition to the RF electrode(s), the susceptor may also include anynumber of other electrodes such as resistive heater electrodes orchucking electrodes. The latter may be implemented using any number ofchucking electrodes and any type of chucking electrode structureincluding monopolar, bipolar, tripolar, interdigitated, zonal and thelike. Similarly, any number or arrangement of heater electrodes can beused including a single heater electrode, or two or more heaterelectrodes may be used for zoned heating and the like. Alternatively,the susceptor may be fabricated as a chuck without heater electrodes orfabricated as a ceramic heater without chucking electrodes. The chuckingand heating electrodes are preferably made of metals such as molybdenumand tungsten and screen printed in a fashion similar to that of the RFelectrode 208.

The vias 210 connect the RF electrode 208 and the robust electrode 212.The vias 210 make contact with the RF electrode 208 at a plurality ofcontact points 218. The contact points 218 are substantially uniformlydistributed across the area of the RF electrode 208. Each via 210comprises a bore 211 in one or more of the ceramic layers 206 _(i)filled with a conducting material 213. An upper end 215 of each via isconnected to the RF electrode 208. A lower end 219 of each via 210 isbonded to the robust electrode 212. FIG. 2 shows that the vias 210 aresubstantially geometrically parallel to each other. Although simple tomanufacture, this is not strictly necessary as long as the vias makeelectrically parallel connections, i.e. side-by-side connections,between the robust electrode 212 and the RF electrode 208. Theseuniformly distributed electrically parallel connections evenlydistribute the RF power over the area of the RF electrode 208.

The susceptor 204 of the present invention depicted in FIG. 2 can befabricated by the method 300 of the present invention shown in the flowdiagram of FIG. 3. The method 300 begins at step 301. In step 302, aplurality of unfired ceramic green tapes are formed that will be used toform a stack of uncured ceramic layers (such as layers 206 _(i)). Thestack of uncured ceramic layers forms the support body upon curing. Thegreen tapes are made from a powdered ceramic such as aluminum nitride oraluminum oxide mixed with an organic binder such as artificial rubber(butadiene), poly-methyl methacrylate, or similar material and cast intosheets. In step 304, bores 211 are punched into selected ones of thelayers. The bores 211 are uniformly distributed over the layers toprovide multiple electrically parallel conducting vias. The bores 211are punched in successive layers such that the bores 211 line up whenthe green tapes are subsequently stacked. The bores 211 may be formedeither before or after the layers have been stacked. Preferably thebores 211 are formed in the green tapes before the layers 206, arestacked.

In step 306, the bores 211 are filled with a conductive material, e.g.,paste containing a metal powder such as tungsten or molybdenum, to formthe vias. The paste completely fills the bores 211. In step 308electrodes (e.g., RF electrode 208) are embedded within the supportbody. For example, the RF electrode 208 may be screen printed onselected green tapes using the same conductive material used to form thevias (i.e., the tungsten or molybdenum paste). Specifically, one or moreRF electrodes 208 are screen printed along with any desired chucking orheating electrodes.

The green tapes are stacked together on top of one another in step 310to form the stack. The stack is pressed at approximately 150° C. in aplaten press in step 312 to produce a green laminate.

The green laminate is then heated in a furnace to remove the organicbinder in step 314. The temperature is typically between 600° and 800°C. during this step. This stage of the sintering may be performed in areducing atmosphere of hydrogen or cracked ammonia. Water vapor isinjected into the reducing atmosphere to provide a controlled partialpressure of oxygen to oxidize the binder while avoiding oxidizing theelectrodes during this step. Next, in step 316, the ceramic body issintered at 1500° to 1600° C. to consolidate the ceramic and the metalto form the susceptor.

The robust electrode 212 is attached to the bottom 209 of the ceramicbody 205 in step 318. The robust electrode 212 can optionally beelectroplated, for example, with nickel to protect it against corrosion.The robust electrode can be formed by screen or stencil printing. Forexample, paste containing a metal powder can be stencil or screenprinted over the exposed lower ends 219 of the vias 210. The body 205 isthen fired in a reducing atmosphere to sinter the powder to a densemetallic structure and simultaneously join the robust electrode 212 tothe vias 210. The pastes are typically loaded with between 10 and 30percent by volume of glass powders (frits) to obtain adhesion to theceramic. Firing typically takes place at temperatures of betweenapproximately 1600° C. to 1800° C. for tungsten and molybdenumcontaining pastes. Copper containing pastes are sintered at about 900°C.

In a preferred embodiment, the ends 219 of the vias 210 are plated withcopper or nickel and the robust electrode 212 is made from molybdenum,copper or tungsten sheet. The robust electrode is attached to the bottom209 of the support body 205 by brazing to the nickel plated ends 219 ofthe vias 210. To do this, the selected braze (e.g., copper-silver alloysor gold-tin alloys) or lead-tin solder can be screened on to the ends219 of the vias 210 to form a plurality of solder dots. The solder orbraze is then melted or reflowed while the robust electrode 212 istouching these solder dots. Joining the robust electrode 212 to thenumerous, well scattered ends 219 of the vias 210 is sufficient torigidly hold the robust electrode 212 during subsequent handling andassembly operations. Thus, it is unnecessary to bond the robustelectrode 212 to the ceramic of the support body 205.

After attaching the robust electrode, the insulator plate 214 isattached to the bottom 209 of the support body 205 in step 320.Attachment can be accomplished by mechanical means such as bolting.Alternatively, the insulator plate can be attached by glass sealing,diffusion bonding or the like. The method then ends at step 321.

The resulting ceramic susceptor 204 incorporates an electrode structurethat exhibits superior transmission of RF power to the substrate andmore uniform distribution of RF power across the substrate than priorart electrode structures. The uniform power distribution leads to moreuniform wafer processing, fewer defective wafers. Consequentlyproductivity is higher, cost per wafer is lower and profitability isincreased.

Although the invention is described in terms of a uniform distributionof multiple parallel electrical contacts any distribution of multipleparallel electrical contacts over the area of the RF electrode thatprovides for a uniform distribution of RF power across the area of theRF electrode is considered within the scope of the invention.

Although various embodiments which incorporate the teachings of thepresent invention have been shown and described in detail herein, thoseskilled in the art can readily devise many other varied embodiments thatstill incorporate these teachings.

What is claimed is:
 1. Apparatus for supporting a wafer in asemiconductor process chamber, comprising: a support body; at least oneelectrode disposed within said support body; a robust electrode disposedbelow said at least one electrode; and a plurality of uniformlydistributed parallel electrical contacts connected between said at leastone electrode and said robust electrode.
 2. The wafer support set forthin claim 1 wherein said support body comprises a plurality of stackedlayers.
 3. The apparatus set forth in claim 1 wherein said robustelectrode is attached to a bottom side of said support body.
 4. Theapparatus set forth in claim 3 further comprising a an insulator plateattached to said bottom side for electrically isolating said robustelectrode.
 5. The apparatus set forth in claim 1 wherein said electricalcontacts are vias further comprising bores filled with a conductivematerial.
 6. The apparatus of claim 5 wherein said material is chosenfrom the group consisting of molybdenum and tungsten.
 7. The apparatusof claim 1 wherein said robust electrode is made from a material chosenfrom the group consisting of copper, molybdenum and tungsten.
 8. A wafersupport for supporting a wafer in a semiconductor process chamber,comprising: a susceptor comprising a ceramic body, wherein said ceramicbody comprises a plurality of layers stacked on top of one another, atleast one electrode embedded within said ceramic body, a robust platedelectrode, made of a metal chosen from the group consisting of copper,molybdenum or tungsten, disposed below said at least one electrode, anda plurality of uniformly distributed parallel electrical contactsbetween said robust electrode and said electrode wherein said electricalcontacts are made of a metal chosen from the group consisting ofmolybdenum and tungsten..
 9. The wafer support of claim 8 furthercomprising an insulator plate attached to a bottom side of saidsusceptor.
 10. A method of fabricating a wafer support, said supporthaving a support body, at least one electrode, a robust electrode and aplurality of electrical contacts connected to said electrode, saidmethod comprising the steps of: forming said support body; disposing atleast one electrode within said support body; forming a plurality ofuniformly distributed parallel electrical contacts to said at least oneelectrode; and disposing a robust electrode below said at least oneelectrode.
 11. The method of claim 10 further comprising the step ofconnecting said robust electrode to said plurality of parallelelectrical contacts.
 12. The method set forth in claim 10 wherein thestep of forming the support body further comprises: creating a pluralityof layers of uncured ceramic; forming a stack of said plurality oflayers; and curing said layers.
 13. The method of claim 12 wherein thestep of forming the plurality of uniformly distributed parallelelectrical contacts comprises: punching a plurality of bores in selectedones of said plurality of layers such that said bores are aligned whensaid stack is formed; filling said bores with a paste containing a metalpowder; and stacking said layers.
 14. The method of claim 10 wherein thestep of disposing said electrode comprises screen printing an electrodepattern on at least one of said plurality of layers.
 15. The method ofclaim 14 wherein said electrode pattern is screen printed prior to saidstep of curing said layers.
 16. The method of claim 10 wherein saidrobust electrode is made from a sheet of metal.
 17. The method of claim10 further comprising the step of attaching a ceramic insulator platebeneath said robust electrode.